The present invention relates to a method for conditioning metallic waste shells made of zirconium or zirconium alloys from the processing of irradiated nuclear reactor fuel elements for noncontaminating final storage.
The fuel elements of nuclear reactors in the majority of cases consist of the actual nuclear fuel and a metallic protective shell which, for use in so-called "thermal" reactors, is usually made of zirconium or a zirconium alloy. The nuclear fuel itself may be uranium oxide, plutonium oxide, thorium oxide, mixtures of these substances, or uranium and plutonium metal.
In the known reprocessing of fuel elements, the fuel elements, either as a whole or after separation of the fuel rods, are mechanically comminuted by means of scissors or shredders according to the almost exclusively employed decomposition process. In a subsequent process step, the metallic or oxidic nuclear fuel is then removed from the shell sections by means of an acid whereby the shredded metallic shell sections remain undissolved. The undissolved shredded metallic shell sections are washed and treated as metallic shell wastes.
The shell waste, consisting essentially of zirconium or a zirconium alloy, is normally radioactive due to neutron activation and contains radioactive fission products and actinides as contaminants. The shell waste must therefore be handled by remote control, according to the known principles for handling substances emitting radioactive and penetrating radiation.
A particular problem in the handling of these metallic shell wastes is the capability of zirconium, and its alloys, to react with oxygen and, at increased temperature, also with the nitrogen of the air, with the development of fire. Fires of radioactive zirconium wastes have been known to occur on several occasions. In addition to the usual undesirable effects of a metal fire, the fire may result in uncontrolled release of radioactivity. In reprocessing systems in which such shell wastes are routinely handled, special measures must be taken to accommodate this potential source of danger. One of these measures which has been used is the treating and storing of shell waste under water. With this method, continuous monitoring of the storage facility and of the water level is necessary and unavoidable. Further, there exists the latent danger of escape of contaminated water into uncontrolled regions. This process is therefore not suitable for permanent storage.
Another known process for conditioning the zirconium containing waste shells is to fix them in a special cement slurry so that a solid block of metal concrete is obtained. Objections have been raised against this process, inasmuch as, depending on the type of shells involved, "gassing" of the concrete has sometimes been noted, which may possibly adversely influence the stability of the bond.